wireline transmission circuit

ABSTRACT

A wireline transmission circuit includes a first circuit that produces a first variable current, a second circuit that produces a first static current, a trans-impedance amplifier that outputs a first analog signal at a first output node in response to the first variable current and the first static current received at a first input node, and a first feedback resistor connected to the first input node and the first output node.

The present application is a Continuation application of and claimspriority to commonly assigned U.S. patent application Ser. No.12/177,377, titled “Improved wireline transmission circuit”, filed onJul. 22, 2008. U.S. patent application Ser. No. 12/177,377 furtherclaims priority to commonly assigned U.S. patent application Ser. No.11/855,081, titled “Improved wireline transmission circuit”, filed onSep. 13, 2007. The content of these applications are incorporated hereinby reference.

BACKGROUND

This application relates to a wireline transmission circuit forpreparing electronic signals to be transmitted in physical transmissionlines.

Wireline communication refers to forms of communications where theinformation is transmitted to the receiver over physical wires, co-axialcables, telephone wires, power lines, or twisted-pair wires. Wirelinecommunication distinguishes itself from other forms of communicationssuch as electromagnetic wireless transmissions or optical signaltransmissions.

A wireline communication device can include a transmission path and areception path. In a transmission path, the data to be communicated isfirst coded and modulated in a digital domain. The modulated digitaldata is converted to analog signals in an analog domain by adigital-to-analog converter (DAC). The analog signals are then amplifiedby a line driver to transmit signals with a large power onto the wiredline (or channel). In some systems, further modulation can beimplemented on the amplified analog signals after the line-driver.

FIG. 1 shows a conventional wireline transmission circuit 100 for thetransmission path of a wireline communication system. The wirelinetransmission circuit 100 includes a digital domain 107 and an analogdomain 108. A digital front-end 101 in the digital domain 107 performscoding and base-band modulation on the digital data to be transmitted.The analog domain 108 includes a D/A Converter 102, a line-driver 103,an optional modulator 104, and a coupling unit 105. The coupling unit105 can couple the amplified signal from the wireline transmissioncircuit 100 to a physical line 106 for wireline transmission.

A challenge to the design and implementation of the line-driver is thatthe line driver is often required to provide large injection power and asmall line impedance, to generate a large voltage magnitude, whilemaintaining excellent linearity in the amplified signals. The largevoltage magnitude and large injection power, however, cannot easily beachieved by a system on chip (SOC) solution. A coupling unit 105 havinga non-unity magnetic winding ratio is hence used to increase the voltagemagnitude of the signals to be transmitted in the physical line 106.

The non-unity magnetic winding ratio in the coupling unit 105 cannegatively impact the power efficiency of the line-driver 103. Theline-driver 103 is required to provide even smaller impedance and largercurrent in the amplified signals to achieve the same injected power inthe physical line 106.

The state-of-the-art CMOS and semiconductor technologies are driven toachieve greater operation speeds in smaller physical dimensions inintegrated circuitry. By reducing the effective gate sizes (width andheight), the transistors can operate at greater speeds (greatertransconductance) and more transistors can be fit in the same physicalspace. The scaling down of the transistors also decreases the break-downvoltages of the transistors and hence the supply voltages to theintegrated circuit have been continuously reduced in deep submicron CMOStechnology.

On the other hand, the line-driver in the wireline transmission circuit,as described above, should operate at the largest supply voltage toachieve power efficiency. To overcome the conflicting requirements,foundries commonly use a mixed-oxide CMOS process in which two differentoxide thicknesses (dimensions, technology generations) are provided on asingle silicon wafer. A deep sub-micron transistor can be placed next toa transistor having larger dimensions, thicker oxide, and thus highersupply voltage on the same silicon substrate. The larger-dimensiondevice is usually used for input and output (I/O) circuits of the chipto sustain high supply voltages and is commonly referred to as an IOdevice.

There is therefore a need to provide a wireline transmission circuithaving improved component integration and smaller device area.

SUMMARY

In a general aspect, the present invention relates to a wirelinetransmission circuit that includes a first circuit that can produce afirst variable current, a second circuit that can produce a first staticcurrent, a trans-impedance amplifier that can output a first analogsignal at a first output node in response to the first variable currentand the first static current received at a first input node, and a firstfeedback resistor connected to the first input node and the first outputnode.

In another general aspect, the present invention relates to a wirelinetransmission circuit that includes a first circuit that can produce avariable current, a second circuit that can produce a first staticcurrent and a second static current, a trans-impedance amplifier thatcan output a first analog signal at a first output node in response tothe variable current and the first static current received at a firstinput node, and to output a second analog signal at a second output nodein response to the variable current and the second static currentreceived at a second input node; a first feedback resistor connected tothe first input node and the first output node; and a second feedbackresistor connected to the second input node and the second output node.

In yet another general aspect, the present invention relates to awireline transmission circuit that includes a first circuit that canproduce a variable current and to direct a variable current to a firstinput node or to a second input node; a second circuit that can producea first static current and a second static current; a trans-impedanceamplifier that can output a first analog signal at a first output nodein response to the variable current and the first static currentreceived at the first input node, and to output a second analog signalat a second output node in response to the variable current and thesecond static current received at the second input node; a firstfeedback resistor connected to the first input node and the first outputnode; a second feedback resistor connected to the second input node andthe second output node. A common-mode voltage at the first input nodesis different from a common-mode voltage at the first output nodes bymore than 0.1V. A common-mode voltage at the second input nodes isdifferent from a common-mode voltage at the second output nodes by morethan 0.1V. A first common-mode correction circuit (CMCC) can inject afirst correction current into the first feedback resistor at the firstoutput node and to inject a second correction current into the secondfeedback resistor at the second output node. A second CMCC can removethe first correction current at the first input node and to remove thesecond correction current at the second input node. The common-modevoltage of at least one of the first input node, the second input node,the first output node, and the second output node is determined by thetrans-impedance line-driver.

Implementations of the system may include one or more of the following.The variable current varies in a range between 0 and Full-Scale (FS) andthe first static current has an amplitude of about −FS/2. The firstcircuit can produce the first variable current in response to an inputsignal. The first circuit can include a current steeringdigital-to-analog converter (DAC) comprising a plurality of currentsources that can produce the variable current. The second circuitcomprises a current steering DAC can include a plurality of currentsources that can produce the first static current. The first circuit,the second circuit, and the trans-impedance amplifier can be fabricatedon a complimentary-metal-oxide semiconductor (CMOS) substrate. The firstcircuit can be fabricated by one or more first oxide layers in a CMOSprocess and the trans-impedance amplifier comprises a transistorfabricated in the CMOS process by a second oxide layer thicker than theone or more first oxide layers. The wireline transmission circuit canfurther include a second feedback resistor connected to a second inputnode and a second output node, wherein the trans-impedance amplifier canfurther output a second analog signal at the second output node inresponse to a second variable current and a second static currentreceived at a second input node. The first circuit can furtherconfigured produce the second variable current and the second circuitcan further produce a second static current. The first variable currentcan vary in a range between 0 and Full-Scale (FS) at the first inputnode and the second variable current varies in a range between −FS and 0at the second input node. A common-mode voltage at the first input nodescan be different from a common-mode voltage at the first output nodes bymore than 0.1V. A common-mode voltage at the second input nodes can bedifferent from a common-mode voltage at the second output nodes by morethan 0.1V. The wireline transmission circuit can further include a firstcommon-mode correction circuit (CMCC) that can inject a first correctioncurrent into the first feedback resistor at the first output node and toinject a second correction current into the second feedback resistor atthe second output node; and a second CMCC that can remove the firstcorrection current at the first input node and to remove the secondcorrection current at the second input node, wherein the common-modevoltage of at least one of the first input node, the second input node,the first output node, and the second output node is determined by thetrans-impedance line-driver. The common-mode voltages at the first inputnode and the second input node can be provided by a first internal loopwithin the trans-impedance amplifier; and the common-mode voltages atthe first output node and the second output node can be provided by aoutput internal loop within the trans-impedance amplifier

Embodiments may include one or more of the following advantages. Anadvantage of the disclosed wireline transmission circuit is that iteliminates the need for external components such as AC couplingcapacitors in some conventional wireline transmission circuit. Theelimination of the external components can also reduce number of pins ina chip, reduce the size of the chip package, and reduce cost. Allcomponents in the disclosed wireline transmission circuit can beintegrated on a single chip (SOC). The integration of all components ona single chip in the disclosed wireline transmission circuit can alsoreduce noise in the output signals comparing to some conventionalwireline transmission circuits whose external components are prune tocouple noise into the wireline transmission circuit.

Another advantage of the disclosed wireline transmission circuit is thatit provides blocks on a chip having a high and a low voltage supplies.Thin oxide devices can be used in a DAC supplied by a lower voltagesource. The line-driver can be implemented by thick-oxide technologywith a high voltage supply. Since the common-mode voltage of the blockscan be changed using the common-mode correction current, voltagesupplies and the associated fabrication technologies can be individuallyoptimized for the high-voltage and the low-voltage blocks.

Yet another advantage of the disclosed wireline transmission circuit isthat it can provide increased power injection to the physical lines (orchannels). The line driver can operate at the high voltage available onthe chip without the risk of damaging the DAC. The output common mode ofthe line-driver can be adjusted almost independently from othercomponents in the wireline transmission circuit. The output common modeof the line-driver can be set at the mid point of the voltage range tomaximize output voltage.

Still another advantage of the disclosed wireline transmission circuitis that the DAC in the disclosed wireline transmission circuit canprovide stable voltages regardless of the amount of currents that theDAC produces. The common modes at the input and the output nodes of theline driver can be externally controlled by for example the digitalfront end.

Although the invention has been particularly shown and described withreference to multiple embodiments, it will be understood by personsskilled in the relevant art that various changes in form and details canbe made therein without departing from the spirit and scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings, which are incorporated in and form a part of thespecification, illustrate embodiments of the present invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 is a block diagram of a conventional wireline transmissioncircuit.

FIG. 2 illustrates an output model for a line driver for a wirelinetransmission circuit.

FIG. 3 is a block diagram for an exemplified wireline transmissioncircuit.

FIG. 4A is a block diagram for an exemplified wireline transmissioncircuit having current steering sources and a trans-impedanceline-driver.

FIG. 4B is a block diagram for another exemplified wireline transmissioncircuit having current steering sources and a trans-impedanceline-driver.

FIG. 5A is a block diagram for a wireline transmission circuit having anexemplified DC current correction.

FIG. 5B is a block diagram for a wireline transmission circuit havinganother exemplified DC current correction.

FIGS. 6A and 6B are block diagrams for wireline transmission circuitshaving DC current correction and common-mode current correction.

DETAILED DESCRIPTION

The power injected onto the line can be looked at as a voltage across animpedance. Referring to FIG. 2, a generic output model 200 for the powerline driver 103 can include a PMOS transistor 201 and a NMOS device 202coupled between a positive voltage supply VDD and a ground voltage. ThePMOS transistor 201 can source current to the output. The NMOS device202 can sink current from the output. The PMOS transistor 201 and theNMOS device 202 drive a line-impedance model 205 that includes animpedance 206 representing the line-impedance and a low impedance node207. The PMOS transistor 201 can drive a current through the impedance206 to create a positive voltage on the physical line. The NMOS device202 can pull current from through 206 to create a negative current onthe line. The node 208 is the input to the gate terminal at the PMOStransistor 201. The node 209 is the input to the gate terminal at theNMOS transistor 202. In some implementations, the node 208 and the node209 can be tied together. The node X201 is the output point of the linedriver 103.

The voltage at the node X201 can swing up or down depending on if acurrent is being sourced or sunk through the line impedance 206. Thevoltage range at the node X201 and the line impedance 206 determine thepower injected into the line. In order to obtain the largest possiblevoltage swing at the node X201, the output model 200 can be set up atVDD/2, halfway between VDD and ground, wherein no current is sourced orsunk, which can be referred to as the common-mode of the amplifiedsignal or the output common-mode of the line-driver. The output PMOStransistor 201 and the NMOS transistor 202 are typically the thickestoxide devices available in a CMOS process to enable a high VDD and allowthe node X201 to have the largest output voltage swing, which asdescribed above allows for better power efficiency and in some cases toeven make power injection possible.

Referring to FIG. 3, a wireline transmission circuit 300 can include acurrent steering DAC 301 which is formed by a bank of current sources302. The current sources 302 can be binary weighted, which incombination can provide a total current strength proportional to thevalue of a binary number. Current steering DACs are suitable forwireline transmission circuits because they can provide greater speedsand resolutions compared to alternative architectures. Most currentsteering DACs can either source (more common, and as shown in FIG. 3) orsink currents but not both.

The current sources 302 are either directed to a positive current path305 or a negative current path 306 under the controls of switches 303which are in turn controlled by the digital front end 101. Resistors307A and 307B are respectively connected to the current paths 305 and306 at nodes X301 and X302 to convert the currents to voltages. Thevoltages at the nodes X301 and X302 can vary between zero and a positivepeak voltage that is determined by the resistance value of the resistors307A and 307B and the peak current. The peak differential voltage acrossthe nodes X301 and X302 is signal, setup and process dependent andcannot be necessarily controlled in all system setups, in particular lowcost solutions aiming for low pin counts. In this setup, the common modevalue of the signal, by nature is half the differential peak voltagei.e. it is simply its full scale divided by two. The common mode can bea relatively low value since it is in reference to ground, and not bewell controlled due to variation of the full scale voltage.

A power line driver 308 includes a voltage line driver 310 having twoinputs separately coupled to resistors 309A and 309B, two output nodes313, and feedback resistors 312A and 312B that control the outputvoltage at the output nodes 313. The resistors 309A and 309B areconnected to coupling capacitors 309A and 309B that can couple ACsignals from the current paths 305 and 306 to the line-driver 308.

The power line driver 308 can be implemented in a high voltage CMOStechnology to provide high output voltage with a large common-modevoltage. AC coupling can decouple the small common-mode voltage (DC) forthe differential voltages between the nodes X201 and X202 and the largecommon-mode voltage for the linear driver 308. The coupling capacitors309A and 309B can act as high-pass filters to allow different DCcommon-mode voltages on the two sides while allowing AC signals to passthrough.

Many wireline transmission circuits are required to provide lowfrequency communications wherein the physical lines may have lowerlosses. The AC coupling capacitors therefore need to be very large toprovide a low cut-off frequency for the high pass filters. Largecapacitors, on the other hand, are hard to implement in CMOS process. Itis hence common for the AC coupling capacitors 309A and 309B to beimplemented as off-chip components. The voltage signals at the nodesX201 and X202 have to come off the chip to be coupled to these externalAC coupling capacitors 309A and 309B. Since the AC coupling capacitors309A and 309B are outside of the chip, the resistors 307A and 307B aswell as the current steering DAC 301 are also commonly implementedoutside of the chip as the line driver 308 which requires a thick oxidelayer. Off-the-chip components can increase the foot print, complexity,and thus manufacturing costs in these wireline transmission circuits.

Referring to FIG. 4A, a wireline transmission circuit 400 includes acurrent steering DAC 401 and a trans-impedance line-driver 410. Atrans-impedance line-driver can also be referred to as a trans-impedanceamplifier. The current steering DAC 401 includes a bank of currentsources 402 that can source a current to a node N401A or a node 401Bunder the controls of switches 403. The current sources 402 can beformed by NMOS or PMOS current source elements, or a combination of PMOSand NMOS current elements. The switches 403 can in turn be controlled bydigital control signals that for example can be sent from a digitalfront end.

The trans-impedance line-driver 410 includes a high-powertrans-impedance stage 413 that includes a pair of input nodes N401A andN401B, and a pair of output nodes. The trans-impedance line-driver 410also includes feedback resistors 414 respectively connected to the nodesN401A and N401B and the outputs of the trans-impedance stage 413. Thetrans-impedance stage 413 has low input impedance at the nodes N401A andN401B and can output a pair of analog signals at the two output nodes toa physical line. The voltage magnitudes of the output signals from thehigh-power trans-impedance stage 413 to the physical line are determinedby the output current of the current steering DAC 401 multiplied by theresistances of the feedback resistors 414. The gain of thetrans-impedance line-driver 410 can thus be determined by the resistancevalues of the feedback resistors 414.

In the present specification, the term “physical line” (also referred toas “wireline channel”) can a range of physical media capable oftransmitting electronic signals, including pair of metal conductors,twisted pair of conductive wires, coax cables, telephone cables,power-line cables, networking cables, or a combination of these media inseries or in parallel.

FIG. 4B shows a wireline transmission circuit 450 which is a variant ofthe wireline transmission circuit 400 as shown in FIG. 4A. The currentsteering DAC 451 includes a bank of current sources 452 that arecontrolled that can sink currents from a node N451A or a node N451Bunder the controls of switches 453.

The wireline transmission circuits 400 and 450 have several advantagesover conventional wireline transmission circuits. Their components canbe integrated on a single chip without the need for large externalcoupling capacitors as in some conventional wireline transmissioncircuits. The package size and the number of pins for the systems can bedecreased. The current steering DACs 401 and 451 that are directlycoupled with the trans-impedance line-driver 410 can also provide higherspeed and improved accuracy than conventional systems.

One drawback associated with the wireline transmission circuits 400 and450 is that the current steering DAC 401 or 451 can either source orsink current, but cannot provide both current sourcing and currentsinking in a single wireline transmission circuit. For example, theoutputs of the DAC 401 to the node N401A and the node 401B are bothpositive currents. The common-mode current between the node N401A andthe node 401B is also in the positive direction (i.e. non zero), whichresults in a DC current offset at the inputs to trans-impedanceline-driver 410. The DC current offset at the input nodes of thetrans-impedance line-driver 410 can in turn produce a large DC voltageoffset through the feedback resistors 414 at the output nodes of thetrans-impedance line-driver 410, which can limit the voltage range thatcan be used by the output signals (i.e. from the negative peak to thepositive peak of the output signals).

In some embodiments, the DC current offset in the wireline transmissioncircuits 400 and 450 can be corrected by an improved design. Referringto FIG. 5A, a wireline transmission circuit 500 includes a currentsteering DAC 501, a trans-impedance line driver 510, and a DC currentsubtracting circuit 520. The purpose of the DC current subtractingcircuit 520 is to substantially remove the DC offsets in the inputcommon modes of the trans-impedance line driver 510 using staticcurrents. The voltage offsets in the output common modes of thetrans-impedance line driver 510 can be below 40% or 20% or 10% or 5%, or2%, or 1%, of the full scale of the output voltage range.

In the present specification, the term “variable current” refers to acurrent that varies in response to input data. A “variable current”typically varies at frequencies of the signal. The term “static current”refers to a biasing or DC current which aimed to be held constant over atransmission frame, but maybe calibrated or adjusted “slowly” at lowfrequencies between frames of communication. Static current elements maybe subject to calibration at low frequency.

The current steering DAC 501 includes a bank of current sources 502 thatcan source a current to a node N501A or a node 501B under the controlsof switches 503. The switches 503 can in turn be controlled by a digitalfront end at a node N505. The trans-impedance line-driver 510 includes ahigh-power trans-impedance stage 513 and feedback resistors 514connected to the inputs and outputs of the trans-impedance stage 413.The trans-impedance stage 513 has low input impedance at the nodes N501Aand N501B. The voltage magnitude of the output signal from thehigh-power trans-impedance stage 513 to the physical line is determinedby the output current of the current steering DAC 501 multiplied by theresistances of the feedback resistors 514.

The DC current subtracting circuit 520 includes DC sinking elements 522and 524. The DC sinking elements 522 can produce a static current tosubtract half full-scale (FS/2) of the current sourced from the currentsteering DAC 501 to the node N501A. The full scale of the sourcingcurrent is defined as the difference between the maximum and the minimumsourcing current that can be produced by the current steering DAC 501.The current entering the node N501A, an input to the trans-impedanceline-driver 510, can be sourcing or sinking with symmetric currentranges on the sourcing and the sinking sides.

Similarly, the DC sinking elements 524 can produce a static current tosubtract half full-scale (FS/2) of the current sourced from the currentsteering DAC 501 to the node N501B. The current entering the node N501B,another input to the trans-impedance line-driver 510, can also besourcing or sinking with symmetric current ranges on the sourcing andthe sinking sides. The DC common-mode current between the node N501A andthe node N501B can therefore be set substantially at zero current,resulting in substantially zero input DC offset.

In some embodiments, the variable current produced at the node N501A bythe current steering DAC 501 can vary between 0 and Full-Scale (FS) andthe variable current produced at the node N501B by the current steeringDAC 501 can vary between −FS and 0, opposite to the current at the nodeN501A. The two oppositely varying currents at the nodes N501A and N501Bcan be referred to as differential variable currents.

In some embodiments, the variable current produced by the currentsteering DAC 501 can vary between 0 and Full-Scale (FS). The staticcurrent produced by the DC current subtracting circuit 520 can haveamplitudes of about −FS/2. Alternatively, the variable current can varybetween 0 and −FS. The static current can be about FS/2. As a result,the currents at the node N501A and the node N501B are differentialvariable currents relative to each other. The current at one of the twonodes can vary from −FS/2 to +FS/2; the current at the other node canvary oppositely from +FS/2 to −FS/2.

The gain of the current steering DAC 501 can be controlled by thedigital front end at the node N505. The full scales of the DAC currentsourced from the current sources 502 to the node N501A and the nodeN501B can vary in accordance to different gain settings. The fill scaleof the sourcing current is defined as the difference between the maximumand the minimum sourcing current that can be produced by the currentsteering DAC 501. The DC sinking elements 522 and 524 can be designed toautomatically subtract half the full scale of the sourcing currents fromthe current steering DAC 501 to the node N501A and the node N501Brespectively. This can be achieved with the introduction of low-speedcurrent correction DACs 523 and 525 in the DC current subtractingcircuit 520 to fine tune the current correction to be precisely at halffull scale. The gains of the low-speed correction current steering DACs523 and 525 are also controlled by digital control signals from thedigital front end. In some embodiments, the low-speed correction currentsteering DACs 523 and 525, and the current steering DAC 501 can becontrolled by the same set of digital control signals. The digital frontend can adjust the gains of the current steering DAC 501 and the gainsof the low-speed current correction DACs 523 and 525 to achieve precisehalf full-scale current subtraction at the nodes N501A and N501B, thusassuring a precisely zero-voltage common mode at the nodes N501A andN501B.

In some embodiments, the current steering DAC 501 is not required toprovide gain functionality in the wireline transmission circuit 500. Insome embodiments, the current steering DAC 501 can have gainprogrammability: the current produced by the current steering DAC 501can change by adjusting biasing references currents or voltages, devicesize, and circuit topologies (parallel or serial connections of thecurrent correction elements). In such a system, a control unit canadjust the DC subtracting value by controlling the DC correction currentin responsive to the DAC gain setting.

In some embodiments, the DC current subtraction circuit can be similarlyincluded in a wireline transmission circuit having a current-sinkingcurrent steering DAC similar to the wireline transmission circuit 450.

The wireline transmission circuit 500 includes the advantages of thewireline transmission circuits 400 and 450 as described above. Inaddition, the input DC offset and output DC error can be substantiallyeliminated. Moreover, the nodes N501A and N501B can have really lowimpedance for the currents from the current steering DAC 501 and the DCcurrent subtracting circuit 520. The voltages at the nodes N501A andN501B can stay stable during signal transmission, which can improveslinearity performance of the current steering DAC (as measured forexample by total harmonic distortion (THD)) and reduce the risk ofcurrent saturation in the DAC elements.

The current steering DAC 501 and the DC current subtracting circuit 520can be implemented using thin-oxide CMOS technologies to achieve greateroperation speed. The current steering DAC 501 and the DC currentsubtracting circuit 520 can be fabricated with one or more oxide layersusing CMOS processes. The different oxide layers can have differentthicknesses. At least portions of the trans-impedance line driver 510can be implemented using the thick oxide CMOS fabrication technologiesto provide high breakdown voltages and to enable high supply voltages.Specifically, the trans-impedance line driver 510 can include one ormore line drivers coupled to the output nodes. These line drivers can befabricated using thick oxide layers while the rest of thetrans-impedance line driver 510 can be built using thinner oxidelayer(s). The nodes N501A and N501B can maintain a constant voltagevalue under operation since these are low impedance nodes. Nodes N501Aand N501B are the common connection points between the current steeringDAC 501 and the trans-impedance line-driver 510. Under normal operationconditions, a DC voltage is required at the node N501A or N501B tomaintain current sinking and sourcing functionality from the currentsteering DAC 501. The thin oxide structure of the current steering DAC501 requires this voltage to be low. On the other hand, the DC voltagesat the nodes N501A and N501B also track the output common-mode voltageof the trans-impedance line driver 510 through the feedback resistors514. As discussed, the output voltages from the trans-impedance linedriver 510 are amplified to large magnitude to achieve maximum powerinjection, which is commonly too high for the current steering DAC 501.A conflict can thus occur at the nodes N501A and N501B between the DCvoltages that are desirable for the current steering DAC 501 and thetrans-impedance line driver 510.

In some embodiments, referring to FIG. 5B, a wireline transmissioncircuit 550 includes a current steering DAC 560, a circuit 570, and atrans-impedance line driver 580. The current steering DAC 560 canprovide a variable uni-polar current that can vary in a range between 0and Full-Scale (FS). The circuit 570 can provide a fixed bipolarizingcurrent having an amplitude at approximately half the full scale, −FS/2.The trans-impedance line-driver 580 has a single-ended input node 575and a single ended output node 590. The input node 575 and the outputnode 590 are connected by a feedback resistor 585. The trans-impedanceline driver 580 receives the currents produced by the current steeringDAC 560 and the circuit 570 as input. The fixed bipolarizing current (at−FS/2) produced by the circuit 570 is added to the variable uni-polarcurrent the current steering DAC 560 at the input node 575 to produce acombined bi-polar current varying in a range between −FS/2 and FS/2.

The Current Steering DAC 560 can be implemented by different designsthat can provide a variable uni-polar current. For example, the CurrentSteering DAC 560 can include a series of current sources that areregulated by switches that can direct different combinations of thecurrents from current sources to the node 575. The currents can also besteered through a current level-shifter element, a high speed currentbuffer, or a cascode device in the current steering DAC.

The circuit 570 can also be implemented by different designs. Forexample, the fixed bipolarizing current in the circuit 570 can beprovided by one or more current sources. In another example, the circuit570 can include a resistor and a regulated voltage source that can applya voltage across the resistor to produce a bipolarizing current havingan amplitude of −FS.

Furthermore, it should be understood that the wireline transmissioncircuit 550 is compatible with a differential trans-impedance linedriver in place of the single-ended trans-impedance line driver 580.Each input node of a differential trans-impedance line driver canreceive a variable bi-polar current in a range between −FS/2 and FS/2,which is the sum of a variable uni-polar current varying in a rangebetween 0 and Full-Scale (FS) and a fixed bipolarizing current having anamplitude of −FS/2.

In some embodiments, the above conflict can be resolved by an additionalcircuitry to distinguish the common-mode voltages at the outputs ofcurrent steering DAC and the output of the trans-impedance line driver.Referring to FIG. 6A, a wireline transmission circuit 600 includes acurrent steering DAC 501, a trans-impedance line driver 510, and a DCcurrent subtracting circuit 520, similar to the wireline transmissioncircuit 500. The current steering DAC 501 sources current into thetrans-impedance line driver 510. In addition, the wireline transmissioncircuit 600 includes common-mode correction circuits (CMCC) 604 and 605.The CMCCs 604 and 605 can inject a correction current from nodes N602Aand N602B to nodes N601A and N601B through the feedback resistors 514.In other words, the correction current is applied to the output nodes ofthe trans-impedance line driver 510 by the CMCC 604 and taken out at theinput nodes of the trans-impedance line driver 510 by the CMCC 605. Noadditional currents are introduced into the current steering DAC 501,the trans-impedance line driver 510, and the DC current subtractingcircuit 520. The common-mode correction currents can introduce DCvoltage shifts between the input and output nodes of trans-impedanceline driver 510. The magnitudes of the common-mode correction currentscan be adjusted to control the difference between the DC voltages at theinput and output nodes of the trans-impedance line driver 510. Themagnitudes of the common-mode correction currents can be pre-set ordynamically controlled by a digital control signal that can be providedby the digital front end. For example, the common modes at the input andoutput nodes of the trans-impedance line driver 510 can be controlled toa voltage difference in a range between 0.1 Volt and 1.5 Volt.

In some embodiments, a common-mode feedback loop circuit can beimplemented to adjust the biasing inside the amplifier to setup apre-defined common-mode voltage at the output nodes of a trans-impedanceline driver. Separately, an input common-mode feedback loop may beimplemented to setup the input common-mode voltage in thetrans-impedance line driver. These common-mode feedback loops can beused in conjunction with the above described techniques to setup thecommon-mode voltages. With the knowledge of the size of the feedbackresistors, the magnitude of the correction current can be chosen toachieve the required difference in DC common-mode voltage between theoutput of the DAC and the output of the trans-impedance line driver. Thecommon-mode feedback loops can determine the absolute voltage values ofthe input node and the output node of the trans-impedance line driver.The common-mode correction current can be produced by the CMCCs inaccordance with the relative voltage values between the input node andthe output node of the trans-impedance line driver.

It should be noted that CMCC for common-mode adjustment and the DCcurrent subtraction circuit can be implemented independently in thedisclosed wireline transmission circuit. The disclosed wirelinetransmission circuit can include CMCC (604 and 605) for DC common-modecorrection without the DC current subtraction circuit (520), and viceversa.

In some embodiments, referring to FIG. 6B, a wireline transmissioncircuit 650 includes similar components to those in the wirelinetransmission circuit 600 except for a current steering DAC 601 sinkscurrent from the trans-impedance line driver 510. A DC currentsubtracting element 620 can subtract half of the full range of thesinking current from the sinking current.

The disclosed wireline transmission circuit may have one or more of thefollowing advantages. An advantage of the disclosed wirelinetransmission circuit is that it eliminates the need for externalcomponents such as AC coupling capacitors in some conventional wirelinetransmission circuit. The elimination of the external components canalso reduce number of pins in a chip, reduce the size of the chippackage, and reduce cost. All components in the disclosed wirelinetransmission circuit can be integrated on a single chip (SOC). Theintegration of all components on a single chip in the disclosed wirelinetransmission circuit can also reduce noise in the output signalscomparing to some conventional wireline transmission circuits whoseexternal components are prune to couple noise into the wirelinetransmission circuit.

Another advantage of the disclosed wireline transmission circuit is thatit provides blocks on a chip having a high and a low voltage supplies.Thin oxide devices can be used in a DAC supplied by a lower voltagesource. The line-driver can be implemented by thick-oxide technologywith a high voltage supply. Since the common-mode voltage of the blockscan be changed using the common-mode correction current, voltagesupplies and the associated fabrication technologies can be individuallyoptimized for the high-voltage and the low-voltage blocks.

Yet another advantage of the disclosed wireline transmission circuit isthat it can provide increased power injection to the physical lines (orchannels). The line driver can operate at the high voltage available onthe chip without the risk of damaging the DAC. The output common mode ofthe line-driver can be adjusted almost independently from othercomponents in the wireline transmission circuit. The output common modeof the line-driver can be set at the mid point of the voltage range tomaximize output voltage.

It is understood that the disclosed hand-held tool driver is compatiblewith other configurations of the electronic components and variations indesigns without deviation from the spirit of the present specification.The above described current steering DACs, trans-impedance line drivers,DC current subtracting elements, and common-mode correction circuits inthe disclosed wireline transmission circuits can be implemented by logicequivalent circuits or elements, or circuits or elements that canperform similar functions. For example, the current steering DACsdescribed in the disclosed wireline transmission circuits can be basedon a thermometer configuration, or a binary weighted system, or acombination of both configurations. The current steering DACs can besegmented in architecture at different points and at different levels.

The trans-impedance element can itself be a line driver as abovedescribed. The trans-impedance element can also be a first-stage linedriver that is connected to one or more second-stage line drivers.Furthermore, the disclosed trans-impedance stage following the DAC alsomay not be a line-driver. In this case, the disclosed trans-impedancestage can be followed by a voltage line-driver in the output stage. Insome implementations, the disclosed wireline transmission system caninclude a CMCC to inject currents into the input nodes of thetrans-impedance line driver and another CMCC to remove the injectcurrents from the output nodes of the trans-impedance line driver.

The disclosed circuits and elements can also be implemented by differentfabrication processes to achieve the disclosed functions. For example,the disclosed wireline transmission circuit can be implemented on asemiconductor substrate having a single oxide layer thickness, ordifferent oxide layer thicknesses. The current steering DAC can be builton a single oxide layer thickness or on a number of different oxidelayer thicknesses. The line driver and the current steering DAC canoperate off the same supply voltage or different supply voltages andeach may use a number of different supplies internally.

Additionally, although the disclosed wireline transmission circuit canallow components to be integrated on a single semiconductor substrate,the disclosed wireline transmission circuit is also compatible withcircuit implementations that include one or more components outside ofthe chip or a number of different chips.

Furthermore, when the trans-impedance line-driver is in a differentialconfiguration, the CMCC circuit can be used to adjust the relativevoltages between the input and output common-modes. The CMCC circuit,however, does not setup the absolute value of the input or output commonvoltage. Common-mode feedback circuits within the amplifier can be usedto setup the output common-mode voltage. Then the CMCC circuit can setupthe input common-mode voltage. Alternatively, the input common-modevoltage can be setup by a common-mode feedback loop, whereas the CMCCcircuit can setup the output common-mode feedback circuit. When an inputand output common-mode feedback circuit is used, a CMCC circuit is notrequired. However the symmetry of the amplifier may require adjustment.

In modern CMOS processes, a number of different oxide thicknesses areavailable on the same substrate. As previously discussed, in theinterest of greater power efficiency, the output of the line-drivershould operate off the largest supply voltage available to the system.For the output transistor to operate off this large supply voltage, theyare preferably implemented in the thickest oxide option available in thefabrication process. Other portions of the wireline transmission circuitmay be implemented in a mixture of different oxide thicknesses.

The present invention is described above with reference to exemplaryembodiments. It will be apparent to those skilled in the art thatvarious modifications may be made and other embodiments can be usedwithout departing from the broader scope of the present invention.Therefore, these and other variations upon the exemplary embodiments areintended to be covered by the present invention.

1-58. (canceled)
 59. A wireline transmission circuit, comprising: afirst circuit configured to produce a first variable current that variesin a range between about 0 and Full-Scale (FS); a second circuitconfigured to produce a first static current that has an amplitude ofabout −FS/2; and a trans-impedance line-driver comprising: atrans-impedance amplifier comprising a first input node and a firstoutput node, wherein the trans-impedance amplifier is configured tooutput a first analog signal at the first output node in response to thefirst variable current and the first static current received at a firstinput node, wherein the first analog signal varies in a first outputrange; and a first feedback resistor connected to the first input nodeand the first output node of the trans-impedance amplifier.
 60. Thewireline transmission circuit of claim 59, wherein a common-mode voltageat the first input node is different from a common-mode voltage at thefirst output node in a range between about 0.1V and about 0.5 V.
 61. Thewireline transmission circuit of claim 59, wherein the first analogsignal has a first voltage offset smaller than 20% of the first outputrange.
 62. The wireline transmission circuit of claim 59, furthercomprising: a first common-mode correction circuit (CMCC) configured toinject a first correction current into the first feedback resistor atthe first output node; and a second CMCC configured to remove the firstcorrection current at the first input node, wherein the first CMCC andthe second CMCC are configured to control the relative differencebetween the common-mode voltages at the first input node and the firstoutput node.
 63. The wireline transmission circuit of claim 62, whereinthe trans-impedance amplifier further comprises a first internal loopconfigured to set an absolute value of at the first input node.
 64. Thewireline transmission circuit of claim 59, wherein the first circuit,the second circuit, and the trans-impedance amplifier are fabricated ona complimentary-metal-oxide semiconductor (CMOS) substrate, wherein thefirst circuit is fabricated by one or more first oxide layers in a CMOSprocess and the trans-impedance amplifier comprises a transistorfabricated in the CMOS process by a second oxide layer thicker than theone or more first oxide layers.
 65. The wireline transmission circuit ofclaim 59, wherein the trans-impedance line-driver further comprises asecond feedback resistor connected to the second input node and thesecond output node of the trans-impedance amplifier, wherein the firstcircuit is further configured to produce the second variable current,wherein the second circuit is further configured to produce the secondstatic current, wherein the trans-impedance amplifier is configured tooutput a second analog signal at the second output node in response tothe second variable current and the second static current received at asecond input node.
 66. The wireline transmission circuit of claim 65,wherein the second analog signal varies in a second output range,wherein the second analog signal has a second voltage offset smallerthan 20% of the second output range.
 67. The wireline transmissioncircuit of claim 65, wherein a common-mode voltage at the second inputnode is different from a common-mode voltage at the second output nodeby more than 0.1V.
 68. The wireline transmission circuit of claim 65,wherein the second variable current varies in a range between −FS and 0at the second input node and the second static current has an amplitudeof about FS/2.
 69. The wireline transmission circuit of claim 65,further comprising: a first common-mode correction circuit (CMCC)configured to inject a second correction current into the secondfeedback resistor at the second output node; and a second CMCCconfigured to remove the second correction current at the second inputnode, wherein the first CMCC and the second CMCC are configured tocontrol the relative difference between the common-mode voltages at thesecond input node and the second output node.
 70. The wirelinetransmission circuit of claim 69, wherein the trans-impedance amplifierfurther comprises a second internal loop configured to set an absolutevalue of the common-mode voltages at the second input node.
 71. Awireline transmission circuit, comprising: a first circuit configured toproduce a first variable current; a second circuit configured to producea first static current; a trans-impedance amplifier comprising: a firstinput node; a first output node, wherein the trans-impedance amplifieris configured to output a first analog signal at the first output nodein response to the first variable current and the first static currentreceived at a first input node, wherein the first analog signal variesin a first output range; a first internal feedback loop configured toset an absolute value of at the first input node; and a second internalfeedback loop configured to set an absolute value of at the first outputnode; and a first feedback resistor connected to the first input nodeand the first output node of the trans-impedance amplifier.
 72. Thewireline transmission circuit of claim 71, wherein the first variablecurrent varies in a range between about 0 and Full-Scale (FS), whereinthe first static current has an amplitude of about −FS/2.
 73. Thewireline transmission circuit of claim 71, wherein a common-mode voltageat the first input node is different from a common-mode voltage at thefirst output node in a range between about 0.1V and about 0.5 V.
 74. Thewireline transmission circuit of claim 71, wherein the first analogsignal has a first voltage offset smaller than 20% of the first outputrange.
 75. The wireline transmission circuit of claim 71, wherein thetrans-impedance line-driver further comprises a second feedback resistorconnected to the second input node and the second output node of thetrans-impedance amplifier, wherein the first circuit is furtherconfigured to produce the second variable current, wherein the secondcircuit is further configured to produce the second static current,wherein the trans-impedance amplifier is configured to output a secondanalog signal at the second output node in response to the secondvariable current and the second static current received at a secondinput node.
 76. The wireline transmission circuit of claim 75, whereinthe second analog signal varies in a second output range, wherein thesecond analog signal has a second voltage offset smaller than 20% of thesecond output range.
 77. The wireline transmission circuit of claim 75,wherein a common-mode voltage at the second input node is different froma common-mode voltage at the second output node by more than 0.1V. 78.The wireline transmission circuit of claim 75, wherein the firstinternal feedback loop is configured to set an absolute value of at thesecond input node, wherein the second internal feedback loop isconfigured to set an absolute value of at the second output node.